Recent investigations have shown that the power density in tube ozonizers can be increased to 25 kW/m.sup.2 if the ozonizer is cooled on two sides and the discharge gap is made very narrow. This imposes high requirements on the mechanical tolerances. Thus, in the German Offenlegungsschrift No. 23 33 311 reference is expressly made to the importance of the tolerances in connection with double cooling.
Traditional ozonizers for industrial applications exhibit a construction such as is shown in German Offenlegungsschrift No. 32 20 018, in particular in FIG. 9. A multiplicity of ozonizer tubes are accommodated in a common shell. Each ozonizer tube consists of an outer metal tube in which an internally metallised glass tube is concentrically disposed with the formation of an annular gap. The metal tubes are welded into a kind of tube base at both ends of the shell. Cooling liquid is fed into the space bounded by the casing of the shell and the two tube bases. In this way the metal tubes (at earth potential), which form the outer electrodes, are cooled.
In addition to narrowly toleranced glass tubes, this type of construction also requires narrowly toleranced metal tubes in order to keep the width of the gap as constant as possible everywhere. In addition, extensive precautions have to be taken to ensure that the metal tubes do not distort when fitted into the tube bases. A further inadequacy of this construction is evident in the fact that different shell sizes have to be made available for different production rates. Thus modularisation is limited by the number of shells and their size.